64

PAINTING MATERIALS

The aqueous adhasives have not proved themselves inferior^. . . They need
further and more exact investigation, but evidently if their flexibility can be some-
what increased and if their composition can include a permanently effective fungi,
cide, the traditional use they have had with paper can be confidently kept in
the future.
Soluble starch can be prepared from starch by treating it with a dilute acid
under controlled conditions. It can be prepared, also, by treating starch with
various oxidizing agents, with glycerine, with enzymes, or with an alkali. Accord-
ing to the conditions, starch may be modified or converted so as to give all degrees
of viscosity or adhesiveness. The mildest treatments give rise to soluble starch;
more complete conversion results in dextrin or sugar (maltose or dextrose). After
the process is completed, the product should be washed and the excess acid or
alkali should be neutralized. The preparations obtained by various methods
are mixtures, in different proportions, of products of the more or less complete
disaggregation of starch. The product dissolves to a clear solution in hot water
and a 2 per cent solution will remain clear or only faintly opalescent for some
days. Soluble starch can be used in the preliminary priming of canvas instead of
animal size. According to Church (p. 95), it is also admirably fitted as a binding
material in water color painting, for the various preparations of soluble starch
become, to a high degree, insoluble in cold water after they have dried.
The preparation of starch from cereal grains dates back to antiquity. A
description is given by Cato, c 170 B.C., in his treatise on Roman agriculture (see
Walton, p. 236). According to Pliny, the process of extracting starch from grain
was discovered by the inhabitants of the island of Chios, Its history in Egypt
dates from very early times. Walton (p. 235) says, * Strips of Egyptian papyrus,
some specimens of which date back to about 3500 B.C., were cemented together
with a starchy adhesive.* There is, however, doubt as to whether starch was used
in the preparation of papyrus. Partington (p. 206) records an observation by
Schubart to the effect that no binder was necessary, the juice of the plant being
sufficient, although a paste from the sifted crumbs of sour bread, or flour with
hot water and vinegar might have been used. It has been reported that a Chinese
document, of the year 312 A.D., was sized with starch. Paste made from the flour
of grain is the traditional adhesive for joining pieces of paper. Cennino Cennini
(Thompson, The Craftsman's Handbook^ p. 65) gives directions for its preparation
and such directions were probably a matter of common knowledge throughout
the Middle Ages and the Renaissance.
Stereochromy (see Water-Glass).
Strasbourg Turpentine (see also Balsam and Oleo-Resia) comes largely from
the conifer, AKt$ excelsa link., which grows in the Vosges Mountains, It is a
balsam easily obtained from blisters in the bark, and it is partially clarified by
sedimentation before being filtered. It differs from Venice turpentine in that it is
coagulated by magnesia and has less color. It is much more expensive, however,